A carburetor for an internal combustion engine in a tool, namely a chain saw, is known from EP 0 688 948 A2. The air required for combustion is inducted from the ambient air through an intake channel via an intake air filter while the fuel to be mixed with the air is supplied from a fuel tank via a carburetor. The carburetor has a control chamber which is delimited by a membrane and filled with fuel. On one hand, the membrane controls an inlet valve in the control chamber and, on the other hand, delimits a compensation chamber which lies on the dry side of the control membrane. A fuel pump is provided in the carburetor housing to move the fuel to the control chamber. From the control chamber, the fuel passes to nozzles through which the fuel enters the intake channel.
In addition, a start-assist device for the internal combustion engine is also provided in the known arrangement. This start-assist device removes the vapor bubbles which develop in the carburetor while the engine is at standstill. The vapor bubbles would otherwise prevent the internal combustion engine from starting at a subsequent attempt. This start-assist device, which is connected between the control chamber and the fuel tank, serves to flood the fuel system. Subsequent start attempts are then made with a filled fuel system which ensures that the internal combustion engine will start quickly with few attempts.
In such tools, the fuel tank is hermetically sealed in order to avoid leaks whatever the work position of the tool. The consumption of fuel due to the operation of the internal combustion engine leads to the formation of underpressure in the fuel tank. This can lead to a strong decrease in fuel volume such that malfunctions occur in the carburetor to be fed or in the internal combustion engine supplied by the carburetor.
A device is known from U.S. Pat. No. 5,526,843 which enables aeration, with the aeration valve being located at an additional opening provided in the fuel tank. A venting valve can also be arranged on the fuel tank to reduce any overpressure developed by heat. Such devices, in which the aeration and venting valves are in direct connection with the atmosphere, are known as "open systems".
A "closed system" for the aeration and venting of a fuel tank is described in U.S. Pat. No. 5,743,240. This system is based on the arrangement of a pressure equalization line together with an aeration and a venting valve between the intake channel and an equalizing vessel connected to the fuel tank itself. The connection of the pressure equalization line to the intake channel or a compensation channel connected to the compensation chamber, forms a closed system. If the tool is used correctly, this system largely prevents the escape of any fuel into the atmosphere.